DC blocking capacitors are often used to eliminate DC offsets into analog circuits. The capacitance value and the resistance of the input set the low frequency cutoff of the composite circuit. In order to obtain a time constant on the order of a millisecond or longer, prohibitively large capacitors are required which are not easily capable of being integrated into CMOS integrated circuits. U.S. Pat. No. 5,875,126, by Minch, et. al, describes an AFGA using a CMOS inverter as a gain element. An electrical schematic diagram of this prior art AFGA 10 is provided at FIG. 1. pFET impact ionized hot electron injection (“IHEI”) is the mechanism used at transistor 12 to inject electrons onto floating gate 14. A tunneling junction 16 tunnels electrons off of floating gate 14 using Fowler-Nordheim tunneling. In the circuit illustrated in FIG. 1, an open-loop inverting amplifier (inverter) includes pFET input transistor 12 and NFET current source 18 which sets the current through pFET 12. With capacitive feedback, the input signal at node Vin is amplified by a closed-loop gain approximately equal to −C1/C2 where C1 is the capacitance of capacitor C1 and C2 is the capacitance of capacitor C2. The maximum gain is limited both by the open-loop gain of the inverter and by the parasitic floating-gate-to-drain overlap capacitance of the inverter.
The complementary tunneling and IHEI processes adjust the floating-gate charge in such a way that the amplifier's output voltage returns to a steady-state value on a slow time scale (on the order of milliseconds to minutes or longer) when the injection current is equal to the tunneling current. If the output voltage is below its equilibrium value, then the injection current exceeds the tunneling current, decreasing the charge on the floating gate; that, in turn, increases the output voltage back toward its equilibrium value. If the output voltage is above its equilibrium value, then the tunneling current exceeds the injection current, increasing the charge on the floating gate; that, in turn, decreases the output voltage back toward its equilibrium value. If the output voltage is equal to its equilibrium value, then the tunneling current and the injection current are the same and the charge on the floating gate stays the same. The circuit behaves like a high-pass filter with a long (≧1 millisecond) time constant. This time constant may be set to be arbitrarily long (e.g., minutes, hours, days, etc.) and may readily be implemented in a CMOS integrated circuit.
FIG. 2 is a voltage versus time plot of the performance of an AFGA in accordance with the circuit of FIG. 1. As can be seen in FIG. 2 at “A”, a step decrease in the Vin signal results in a downward adaptation of the Vout signal. This adaptation rate is controlled by the tunneling process. At “B”, a step increase in the Vin signal results in an upward adaptation of the Vout signal. This adaptation rate is controlled by the IHEI process and does not match the adaptation rate of the tunneling process. Thus, in this implementation, positive adaptation does not match negative adaptation.
The circuit of FIG. 1 represents a relatively simple circuit. It implements a single gain stage which thus limits the ability to control the voltage on the floating gate. Performance is also limited in that the mechanisms used to raise and lower the charge on the floating gate are not the same and exhibit different time constants. For example, the output bias voltage rises according to the dynamics of the IHEI process and it falls according to the different dynamics of the tunneling process. The Fowler-Nordheim tunneling mechanism used in FIG. 1 typically requires that Vtun be in excess of about 10 Volts whereas the typical CMOS supply voltage is less than about 3 Volts. The implementation is a single-ended, single input, inverting configuration.
The feedback path in CMOS integrated amplifiers typically uses resistors. The resistors can take the form of physical resistive elements, or can take the form of switched capacitors. Physical resistors provide continuous-time feedback and thereby allow the construction of wideband amplifiers, but the resistance of such resistors is typically relatively small so that amplifiers that use them consume relatively high power. The resistance value of switched capacitors can be large, solving the power problem, but the amplifier bandwidth is limited by the capacitor switching frequency (and does not typically exceed ⅕ of the capacitor switching frequency). A mechanism for capacitive feedback without switching is therefore desired, one which utilizes low power and achieves continuous-time wideband operation. Similarly, improved performance through matched time constants, multiple gain stages, relaxed voltage requirements, multiple inputs, differential operation and/or non-inverting architecture is desirable.